Abstract Bacteria typically live in complex sociomicrobiological communities, often as biofilms, on surfaces as diverse as water pipes, ship hulls, plant roots, insects, shellfish, indwelling medical devices, and mucosal surfaces. Biofilm growth in humans can cause or exacerbate disease, and biofilm growth in environmental niches can facilitate transmission of pathogenic bacteria to humans and other animals. Understanding how bacteria recognize, cooperate and compete with their neighbors in diverse environments is critical for developing strategies to control microbiological community composition, to prevent biofilm development, and to eliminate pre-existing biofilms and their consequent diseases. Contact-Dependent Growth Inhibition (CDI) is a phenomenon in which bacteria use the toxic C-terminus of a large exoprotein to kill or inhibit the growth of neighboring bacteria upon cell-cell contact. Production of a small immunity protein protects bacteria against CDI. Using the Gram-negative bacterium Burkholderia thailandensis as a model, we have discovered that in addition to using CDI system proteins to kill their neighbors, bacteria can use these proteins for signal transduction, causing a change in gene expression that leads to the production of cooperative behaviors, such as biofilm formation, when neighboring bacteria are recognized as ?self?, a phenomenon we call CDS (for contact-dependent signaling). We recently discovered that the genes encoding the CDI system proteins in B. thailandensis (bcpAIOB) are located on a large mobile element that defines a new class of transposon. We showed that this transposon moves using a copy-out-paste-in mechanism, that the copy-out step, which results in the formation of a large (210 kb), circular, extrachromosomal ?megacircle?, requires the activity of the BcpA exoprotein, and that megacircle formation is required for CDS phenotypes. We now plan to determine the molecular mechanisms by which the bcpAIOB genes and the proteins they encode contribute to megacircle formation, the molecular mechanisms by which megacircle formation leads to gene expression changes resulting in cooperative behaviors, and the role this system plays in the development of sociomicrobiological community development. Understanding the function of these systems at the molecular level may lead to the development of new antibiotics, new approaches to blocking biofilm development and biofilm-mediates diseases, and new approaches to blocking transposon-mediated spread of antibiotic resistance.